1. Field of the Invention
This invention relates to a method and apparatus for supporting a mine roof, and more particularly, to a bendable roof bolt having a preselected length and a uniform cross section which may be bent at a predetermined location along the roof bolt preselected length to permit a portion of the roof bolt to be inserted in a bore hole and thereafter partially straightened to allow the remainder of the roof bolt to be advanced in the bore hole.
2. Description of the Prior Art
It is well known in the art of mine roof support to insert a roof bolt in a bore hole drilled in a mine roof and thereafter anchor the roof bolt within the bore hole to reinforce the unsupported rock formation above the roof. However, due to the unstable condition of the rock strata above the mine roof, it is sometimes necessary to drill a bore hole in the mine roof to a depth greater than the available vertical working distance between the mine floor and the mine roof in order to reach rock strata capable of sufficiently anchoring the roof bolt within the bore hole.
When this situation occurs, it is necessary to provide a roof bolt having an overall length greater than the available vertical working distance between the mine floor and mine roof. Since the roof bolt has an overall length greater than the vertical working distance between the mine floor and mine roof, the roof bolt must be bent to reduce its overall length and permit an end portion of the roof bolt to be inserted in the bore hole. After the roof bolt is bent and the end portion inserted in the bore hole, the roof bolt is straightened to allow the remainder of the roof bolt to be advanced in the bore hole.
Bendable roof bolts are presently commercially available for use in locations where a roof bolt having an overall length greater than the available vertical working distance between the mine floor and mine roof is required to reinforce the unsupported rock formation above the roof. In one commercial embodiment of the presently available bendable roof bolt, the roof bolt body includes a notched-out area positioned at a preselected location along the length of the roof bolt to reduce the diameter of the roof bolt and thus facilitate bending before the roof bolt is inserted in the bore hole. In another commercial embodiment, the roof bolt includes a pair of notched-out areas, the pair of notched-out areas being located opposite from each other around the circumference of the roof bolt. The pair of notches are positioned opposite from each other to reduce the diameter of the roof bolt and further facilitate bending of the roof bolt before insertion in the bore hole.
Although bendable roof bolts are commercially available, these roof bolts are expensive to produce since forming a notch in the body of the roof bolt requires either heating and pressing the roof bolt, milling the roof bolt body, or reduced rolling the roof bolt at the area of the notch. Reduced rolling the roof bolt to form the notch may be done by either a hot or cold rolling process. In addition, since the diameter of a notched roof bolt is reduced at the location of the notch, the mechanical properties of the notched roof bolt are a function of the roof bolt diameter at the location of the notch. As a result, in order to provide a notched roof bolt having pre-selected mechanical properties, a roof bolt must be provided having a diameter larger than required to compensate for the reduction in diameter at the notch.
Therefore, there is a need for a bendable roof bolt having a uniform cross section which does not require notching of the roof bolt body to facilitate bending of the roof bolt. Since the roof bolt has a uniform cross section, the roof bolt may be made from a smaller diameter rebar material than the presently commercially available notched roof bolt and still exhibit mechanical properties equivalent to that of the larger diameter notched roof bolt. Since the uniform cross section roof bolt has a smaller diameter than the notched roof bolt, it is lighter in weight and is also easier to bend.